gradle: bump android plugin version to 2.1.2

Bugfix release, no changelog on developer.android.com.

build.gradle

@@ -3,7 +3,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.1.0'
+        classpath 'com.android.tools.build:gradle:2.1.2'
         classpath files('libs/gradle-witness.jar')
     }
 }